Nitrile rubbers (NBR rubbers) are products of the copolymerization of 1,3-butadiene and acrylonitrile, generally having an acrylonitrile content within the range of 15 to 50% by weight. According to the known art, these rubbers are formulated with plasticizers, cross-linking agents, normal fillers and additives and subjected to vulcanization to give end-products whose characteristics widely depend on the composition of the NBR rubber. In particular, in vulcanized products the values of resistance to liquid fuels and solvents, and the resistance to abrasion and permeability of gases, increase with the increase of the acrylonitrile content. On the other hand, with a decrease in the acrylonitrile content the characteristics at low temperature and resilience improve. These vulcanized products are consequently used in technological fields which require high performances, such as the car industry, the manufacture of gaskets, piping and oil seals. For a review of NBR rubbers, their production, formulation and vulcanization, reference can be made to Kirk-Othmer, Encyclopedia of Chemical Technology, Ed. 1982, Vol. 8, page 184 onwards and Vol. 20, page 365 onwards.
The functions of the plasticizers used in vulcanizable compositions of NBR rubbers basically consist in increasing the performance of the formulate at low temperatures, improving the workability in relation to the different transformation and moulding techniques, lowering the cost of the formulate, in that the addition of the plasticizer enables a greater quantity of fillers to be incorporated, and modifying the resistance to solvents of the vulcanized products.
A group of plasticizers, for general use, which are suitable for the purpose, is that of dialkyl esters of phthalic acid. When specific characteristics are required, aliphatic plasticizers are used, which are relatively more valuable, such as the dialkyl esters of adipic acid, sebacic acid (for example di-2-ethyl-hexylsebacate, DOS) and azelaic acid, or the polymeric esters of said acids, in particular adipic acid, especially when vulcanized products are required which are particularly resistant and impermeable to the action of oils and solvents.
The disadvantage of the above plasticizers basically consists in their high cost and consequently the necessity is felt in the art for more economical plasticizers.